1. Field of the Invention
This invention relates generally to a method for increasing the surface area and plastic properties of lime, in particular hydrated limes. More specifically, this invention relates to a method of chemically modifying the quicklime hydration reaction with a chemical modifier to bring about a much higher surface area lime hydrate.
2. Description of the Prior Art
Hydrated lime is produced when quicklime (calcium or magnesium oxide) is reacted with water. This reaction is exothermic with approximately 500 Btu/lb quicklime being evolved. Also, the volume of the lime particles increases as much as 94% for CaO and 119% for magnesium oxide. The most likely explanation for the mechanism of the hydration process is that hydration starts at the most active sites of the lime crystal, causing a volume increase and a stress on the crystal. The stresses which occur cause the lime particles to break open further, which allows water to reach additional active sites and continue the hydration process. This activity continues until all the active lime is hydrated. As the active sites on the lime crystals decrease, the hydration activity slows down and the evolution of heat stops.
Higher surface areas of chemicals are known to increase the chemical activity of such chemicals. As a result, higher surface area is an advantage in many applications. For example, where lime is being used for its plasticizing properties, greater surface area allows decreased amounts of lime to be used and still retain the desired degree of workability.
Both relative activity of the quicklime and the temperature of the hydrating water will determine the surface area of the resulting hydrated lime. The chemical activity of hydrated lime is dependent upon particle size, and the smaller the crystal size the greater the surface area of the hydrated lime. Despite the fact that higher water temperature is generally advantageous, one limit on increased temperature is that during the hydration process the water must remain in the liquid state to produce the very small hydrate particles that give high surface areas. Water in the superheated or vapor state (i.e., live steam) produces very low surface area hydrates. Such hydrate produced under uncontrolled temperature conditions referred to herein as "high temperature hydrate" has been found to be denser, quicker settling as a hydrate suspension, less plastic in the form of putty, slower reacting in causticizing processes and in general, inferior to "controlled temperature hydrate" formed under controlled temperature conditions.
This aspect of lime hydration is discussed, for example, in the article "Hydration Control For Production Of Superior Calcium Hydroxide And Hydration Of Magnesium Oxide", Azbe Corporation, February, 1949. The Azbe article discusses a mechanical device and process for controlling the temperature of the hydration water within the reactor vessel. The mechanical apparatus and method of use is difficult and expensive.
It is accordingly an object of this invention to produce a hydrated lime, either high calcium or dolomitic, that is characterized by increased surface area over traditional lime hydrates, in which a chemical modifier is used to control the temperature of the hydration water in order to provide increased surfaces area in the resulting hydrate.
Another object of this invention is to produce a hydrated lime that will increase the lime's ability to hold water and become more plastic.
Another object of this invention is to produce a hydrated lime that can be used in mortars in smaller proportions than used in general practice and still retain the workability required by codes or standards.
Another object of this invention is to increase the hydration reaction of magnesium type, i.e., dolomitic, quicklimes.
Another object of the invention is to provide a modifier for a dolomitic quicklime which catalyzes the magnesium hydration reaction to less than one hour from a normal 24 hour reaction rate.
Another object of this invention is to provide a method of producing dolomitic hydrated lime suitable for mortars without having to use pressure methods of hydration.
Another object of this invention is to provide a method of producing a very high surface area hydrated lime slurry that can be located near a fossil fuel boiler plant and provide a highly active acid gas sorbent for spray scrubbers.
Another object of this invention is to provide a method of producing a very chemically reactive hydrated lime slurry that can be located near process chemical plants, such as recaustisizing operations, and provide such slurries for chemical processes.
Another object of this invention is to provide a hydrated lime that will increase the mortar life of premixed mortars, such as called "ready mix mortars", in which an additive is mixed with mortar mixes to delay the set time of the mortar for one to two days.
Another object of this invention is to provide a hydrated lime that will reduce the amount of the special additive used in ready mix mortars.